1. Field of the Invention
This invention lies in the area of DC to DC converter devices and, more particularly, converter devices adapted to provide efficient low power drain conversion of extremely low level DC power sources.
2. Description of the Prior Art
DC to DC converters have been widely used in the prior art for conversion of a low level source to a higher level DC source. Generally, such prior art converter circuits employ some sort of chopper, or oscillator, for effectively producing an AC signal, which signal may be amplified to a desired amplitude, following which it is rectified to re-establish the DC signal at the desired level. A large number of prior art designs are available, providing varying degrees of voltage amplification and regulation efficiency. However, all of these prior art circuits are designed with the assumption that the starting DC signal level is sufficient to enable unaided operation of the active device or devices being utilized. Power efficiency, or current drain in the converter unit, may or may not be a consideration in these circuits.
In spite of the prior art history of converter devices, there remain certain applications which require performance characteristics that have heretofore not been available. For example, in the field of cardiac pacing devices, referred to hereinafter as pacers, DC to DC converters are necessary where the power source provides a voltage which is too low to act as a supply for the logic and stimulus generating circuitry. In an extreme example, where nuclear powered sources are being utilized, the delivered voltage from the nuclear source is substantially less than 1 volt, and the available power is substantially less than one milliwatt. In such a case, conventional prior art converters are not able to provide the desired conversion, both because conventional designs don't provide the required oscillator startup characteristics and the converter efficiencies are too low to provide the required output power. A typical nuclear power source will have a voltage throughout its predicted lifetime which will vary between about 0.60 volts and 0.40 volts, with an internal impedance of about 100 to 300 ohms. In order to have an available voltage of 4 to 5 volts, and supply 20 to 25 ua, it is clear that a reasonably good converter efficiency is required. Also, particularly for an application such as a cardiac pacer, expense and reliability are important criteria, such that the simplest possible design is the best design.
For the nuclear power source characterized hereinabove, it is clear that at oscillator startup the starting current will be only a matter of several ma, even assuming zero input impedance at startup. Consequently, any active device, or devices, chosen for inclusion in the oscillator configuration must be characterized by having substantially negligible impedance when conductive, so as not to limit the starting current even further. The device also must be characterized by being switchable with a rather low voltage signal. It is known, of course, that oscillator startup may be achieved by designing high loop gain, but it is also understood that such overly high loop gain contributes to power loss and power inefficiency. Consequently, the application calls for a circuit design including a very low resistance starting current path and a configuration of one or more active devices providing a low switching level so that a relatively low loop gain oscillator can be employed. By selecting an active circuit configuration which presents an extremely low resistance when in the on state, and by utilizing a feedback circuit designed for efficient device operation, a highly power efficient oscillator is achieved which is capable of reliably starting when powered by an extremely low voltage level such as a nuclear power source. The addition of a low current drain stabilizer circuit which interfaces with the converter circuit provides means for efficiently and reliably delivering required power at a predetermined voltage.